An idyllic summer’s day, a peaceful waterfront scene, a picnic, a happy family. But since this four-parter is called The Drowning, it seems obvious what’s going to happen to that cute kiddie with the headful of bouncing curls and the scar on his cheek while the oblivious adults drink and chat.

However, nine years after this prologue, the picture isn’t altogether clear. The body of four-year-old Tom never resurfaced from its presumed watery grave, and a mysterious couple in the vicinity at the time failed to come forward despite numerous appeals. Tom’s parents, musician Ben and garden landscaper Jodie, have split up, leaving Jodie to rattle around a huge bare house living what she sardonically calls a “Scandi lifestyle”. One day Jodie spots a boy in school uniform with curly hair and a guitar case. He is just the age Tom would be now, and also, she confirms when she gets close enough, bears a scar.

Teasing questions quickly arise about the fallibility of memory and the selectiveness of evidence. Unlike Jodie, Ben never believed Tom was abducted, but it might not be because he’s the more rational of the two.

The pursuit of Daniel, as he is called, takes Jodie (Jill Halfpenny) into a much more ambivalent zone than just bereaved mother. She manages to inveigle herself into the boy’s school and life, meeting his scary architect father, Mark (Rupert Penry-Jones), who seems to have plenty of secrets of his own. At this point there’s so much creeping and snooping that the most inattentive parent or headteacher would be yelling for the police. How come Jodie always seems to be hanging around in the corridor just when Daniel emerges from class? Fortunately the school has lots of balconies and viewing points for a suspicious staff member (Roisin O’Neill, amusingly furtive) to keep watch on the interloper.

In a drama so concerned with genetic inheritance, it’s appropriate that Cody Molko, full of pathos as guitar-strumming Daniel, seems to have got his musical and performing capabilities from his father, Brian Molko of the band Placebo. He deploys just the right level of bashful cheek as Daniel, who’s both flustered and flattered by Jodie’s peculiar attentions. Halfpenny’s intense performance — part Medea, part governess in The Turn of the Screw — gives her character’s interactions with the unsuspecting teen a nail-biting edge. Director Carolina Giammetta piles on the tension to almost absurd levels, but working out whether Jodie’s the sane one in a deluded world, or about to lose it with the garden shears, is enough to keep you gripped.

★★★★☆

On Channel 5 from February 1-4 at 9pm

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